Temperature-staged cryogenic apparatus of stepped configuration with adjustable piston stroke

ABSTRACT

A temperature-staged cryogenic refrigeration apparatus of stepped configuration wherein a high-pressure fluid is introduced into and then exhausted as low-pressure fluid from two or more variable-volume refrigeration chambers defined by moving pistons. The refrigeration chambers are of decreasing diameter and deliver refrigeration at decreasing temperature levels. One or more of the pistons defining the variable-volume refrigeration chambers is linked to its adjacent piston through variable piston linkage means so that the stroke length of the piston as well as its diameter may be used to adjust the ratios of maximum volumes between the refrigeration chambers.

United States Patent Bamberg et al. 1 Apr. 9, 1974 15 TEMPERATURE-STAGED CRYOGENIC 3,416,307 12/1968 Wallis 62/6 APPARATUS OF STEPPED 3.416.308 12/1968 Livezey CONFIGURATION WITH ADJUSTABLE 3,673,809 7/1972 Bamberg 62/6 PISTON STROKE Primary ExaminerWilliam J. Wye Inventors? Walter Bambel'g, stoughton; Attorney, Agent, or FirinBessie A. Lepper James A. ONeil, Bedford, both of Mass- 57 ABSTRACT Asslgneei Cryogenic Tetihnology, -a A temperature-staged cryogenic refrigeration appara- Waltham Masstus of stepped configuration wherein a high-pressure [22] Filed: N0 21,1972 fluid is introduced into and then exhausted as lowpressure fluid from two or more variable-volume rel PP 308,409 frigeration chambers defined by moving pistons. The refrigeration chambers are of decreasing diameter and [52] US. Cl. 62/6, 60/24 deliver refrigeration at decreasing temperature leve|5- 51 1m. (:1. F25b 9/00 one Or more Of the Pistons defining the variable- [58] Field of Search 62/6; 60/24 volume refrigeration chambers is linked to its adjacent piston through variable piston linkage means so that [56] References Cited the stroke length of the piston as well as its diameter UNITED STATES PATENTS may be used to adjust the ratios of maximum volumes between the refrigeration chambers. 2.508.315 5/1950 Weenen 60/24 3.218315 11/1965 Chellis 37 Claims, 23 Drawing Figures PATENTEUAPR 9 1914 SHEEI 1 [If 6 PATENTEBAPR 8 I974 sum 3 or e Fig.1O Fi .11

A-INLET CLOSED B-EXHAUST X OPEN C-EXHAUST CLOSED i F LENGTH OLUME) wmDwmmmL INLET OPEN-D STROKE Fig.13

9ATENTEIJ 9 1974 PRESSURE STROKE LENGTH (VOLUME) Fig.2O

Llll AK IYAY SHEET 5 BF 6 TEMPERATURE-STAGED CRYOGENIC APPARATUS OF STEPPED CONFIGURATION WITH ADJUSTABLE PISTON STROKE This invention relates to temperature-staged cryogenic refrigerators and more particularly to temperature-staged cryogenic refrigerators of a stepped configuration adapted to deliver refrigeration at more than one temperature level. The basic apparatus principles are applicable to cryogenic refrigerators operating on the cycle described in U.S. Pat. No. 2,906,101 and more particularly to the form of that cycle described in U.S. Pat. No. 2,966,035. The invention is also applicable to apparatus for developing refrigeration through the use of the Stirling and the Vuilleumier cycles.

In constructing a temperature-staged cryogenic refrigerator having two or more refrigeration chambers it is of course necessary because of mass flow considerations to construct each refrigeration chamber to havev a smaller volume than that delivering refrigeration at the next higher temperature level. Superimposed upon this requirement for decreasing refrigeration chamber volumes are such considerations as the achievement of simplicity of construction, the attainment of maximum regenerator efficiency and the minimization of heat losses, particularly those due to heat leaks brought about through conduction and piston motion. The most satisfactory design which meets these requirements is a stepped configuration, that is a refrigerator having a housing which is constructed in the form of hollow cylindrical sections of decreasing diameters suitably sealed to define a fluid-tight enclosure and rigidly joined pistons of varying diameter adapted to experience reciprocal motion in the housing sections. The pistons through their motion define refrigeration chambers of decreasing diameter thereby providing the decreasing refrigeration chamber volumes desired. Thus in this widely used stepped configuration, since the pistons are rigidly linked and all undergo the same length of stroke, the chamber diameter is the only design parameter which may be changed to control the volume ratio of the two or more refrigerations chambers.

It is not feasible, however, to arbitrarily vary the diameter of the colder refrigeration chambers without regard to several limiting factors. For example, in a stepped-configuration refrigerator wherein the regener'ators are located within the pistons (and this is generally the case) the regenerator pressure drop and the thermal performance of the regenerator is at least in part determined by the regenerator diameter (and hence the piston diameter) as well as the length of the regenerator. This fact in turn places relatively rigorous limitations on the extent to which the pistons defining the lower-temperature refrigeration chambers may be reduced in diameter. This then leaves as an alternative to decreasing piston diameter, the control of the stroke length for those pistons defining the volumesof the lower-temperature refrigeration chambers. Since-there is provided a single drivingmeans for these piston sections formed of decreasing diameters, the previously used rigid mechanical linkagesbetween piston sections is not suitable to vary the piston stroke lengths.

The apparatus of this invention makes possible the adjustment of piston stroke length by providing linking means for the piston sections which permit engagement between the piston sections in series and thereby causes the largest diameter piston to impart reciprocal motion to one or more smaller diameters pistons through a stroke length less than that experienced by the largest diameter piston. It has been found that by varying the ratio of stroke lengths it is possible to vary the temperature and refrigeration capacity for each stage within certain limits, to optimize overall refrigerator performance by adjusting relative piston displacements, and to decrease the losses associated with the lower-temperature regenerators while still obtaining the benefits of previously determined optimal design considerations such as minimum conduction and motional heat leak losses and simplicity of overall construction.

It is therefore a primary object of this invention to provide an improved temperature-staged cryogenic refrigerator of stepped configuration. It is another object to provide a refrigerator of the character described wherein the volume of the refrigeration chambers operating at the lower temperature levels may be readily optimized by changing the length of stroke of the reciprocating piston section defining the volume. It is yet another object to provide a cryogenic refrigerator which is capable of attaining lower temperatures than one of equal size and configuration constructed with'a stepped piston, the sections of which are rigidly linked. This invention has for a further object the provision of a stepped, temperature-staged cryogenic refrigerator in which the mass flow may be optimized for each stage, the regenerator efficiency may be increased, and the PV diagram for any stage optimized for a desired performance. Other objects of the invention will in partbe obvious and will in part be apparent hereinafter.

The cryogenic apparatus which achieves the objects of this invention is one which is adapted to introduce a high-pressure expansible fluid from a high pressure fluid source into two or more refrigeration chambers of variable volumes operating at different temperature levels and to discharge expanded low-pressure fluid therefrom to a low-pressure reservoir. The fluid is conveyed to the refrigeration chambers through a fluid flow path incorporating regenerators and is returned from the chambers back through the same flow path. The refrigeration chambers have diameters which decrease with decreasing temperature levels and their variable volumes are defined by reciprocating pistons which are so linked as to provide engagement between the pistons in series such that the largest diameter piston imparts reciprocal motion to the other piston or pistons through predetermined and controlled stroke lengths, these stroke lengths being less than that experienced by the largest diameter piston.

The apparatus of this invention is applicable to devices designed to operate on the cycles of U.S. Pat. Nos. 2,906,101 and 2,966,035 wherein the highpressure fluid source and low-pressure fluid reservoirs are external of the refrigerator and are connected through valved-controlled lines, as well as on the Stirling cycle such as illustrated by U.S. Pat. Nos. 2,657,553 and on the Vuilleumier cycle such as disclosed inU.S. Pat. Nos. 1,275,507.

The use of a variable linkage between pistons has been applied to in-line refrigerators wherein each pistonmoves within its own separate concencentric housing that extends up into the warm end of the refrigerator. (See U.S. Pat. No. 3,673,809.) This makes it possible to use seals only on the warm end. The apparatus of U.S. Pat. No. 3,673,809 is suitable for refrigerators structing larger refrigerators where relatively large heat losses can be tolerated.

In the apparatus of U.S. Pat. No. 3,673,809 the pistons are all of essentially of the same diameter, a fact which means that stroke length is the only design parameter which can be adjusted to achieve desired ratios between the refrigeration chamber volumes. Therefore the construction and operation of a small inline refrigerator such as described in U.S. Pat. No. 3,673,809 designed to attain very low temperatures and relatively high efficiencies present almost insurmountable problems. This is due to the fact that since the diameters of the lower-temperature refrigeration chambers must be essentially as large as that of the refrigeration chamber at the highest temperature, the stroke lengths of the piston defining the lower-temperature chambers must be extremely short, a fact which presents adjustment and control problems. Moreover, high thermal losses due to excessive conduction through the extended chamber surface and to large motional heat leaks through the extended piston surfaces materially detract from the realization of an efficient cryogenic refrigerator, particularly in small sizes, capable of delivering refrigeration at low temperatures, e.g., below lK.

The apparatus of this invention makes it possible to balance the factors of piston and cylinder diameters with piston stroke lengths to obtain optimum operational performance for any sized refrigerators and particularly those delivering up to about watts refrigeration at IOK or lower.

The invention accordingly comprises the features of construction, combination of elements and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken inconnection with the accompanying drawings in which FIG. 1 is a somewhat simplified longitudinal cross section of a two-staged refrigerator, constructed in accordance with this invention to operate on the cycle of U.S. Pat. No. 2,966,035, along with its attendant components;

FIG. 2 is a modification of the refrigerator of FIG. 1 wherein the regenerators are located external of the refrigerator housing; 1

FIG. 3 is a somewhat simplified longitudinal cross section of a three-staged refrigerator constructed in accordance with this invention to operate on the cycle of U.S. Pat. No. 2,966,035 along with the components of a Joule-Thomson loop adapted to use the refrigeration developed to liquefy helium;

FIG. 4-7 are detailed cross sections of four embodiments of variable linkage means between pistons usable in the refrigerators of FIGS. 1-3;

FIGS. 8ll illustrate the steps involved in the operation of the variable linkage of FIGS. 4 and 5;

FIG. 12 illustrates a variation in the last step of operation when the variable linkage embodiment of FIG. 7 is used.

- FIG. 13 is a PV diagram for a typical cryogenic refrigerator operating on the cycle of U.S. Pat. No. 2,966,035 and incorporating early cutoff of the supply of high-pressure fluid into the refrigerator;

FIG. 14 is a PV diagram for the colder refrigeration chamber of a two-staged refrigerator constructed in accordance with this invention and using the linkage of FIG. 4 or 5;

FIG.,15 is a PV diagram for the colder refrigerator chamber of two-staged refrigerator constructed in accordance with this invention and using the linkage of FIG. 7;

FIG. 16 is a detailed cross section of an embodiment of this invention wherein the variable linkage between pistons is a combination of surface engagement and restraining means affixed to the refrigerator housing;

FIGS. 17-19 illustrate the steps involved in the operation of the refrigerator embodiment of FIG. 16;

FIG. 20 is a PV diagram for the colder refrigerator chamber of a two-staged refrigerator constructed in accordance with this invention and using the linkage of FIG. 16;

FIG. 21 is a somewhat simplified longitudinal cross section of a two-staged refrigerator constructed in accordance with this invention to operate on the cycle of U.S. Pat. No. 2,906,101;

FIG. 22 is a somewhat simplified longitudinal cross section of a two-staged refrigerator constructed in accordance with this invention to operate on the Stirling cycle; and I FIG. 23 is a somewhat simplified longitudinal cross section of a two-staged refrigerator constructed in accordance with this invention to operate on the Vuilleumier cycle of U.S. Pat. No. 1,275,507.

It should be noted in the following description of the apparatus of this invention such words as warm and cold are relative, and the use of up and down are for convenience in describing one exemplary orientation of the apparatus since the devices under consideration may generally be operated in any orientation. To simplify the discussion and to make the description of the apparatus applicable to apparatus formed in two, three or more stages, the terms largest-diameter, smallest-diameter etc. as applied to pistons, displacers and refrigeration chambers will generally be used even through only two stages are involved. It will also be convenient to refer to the movable body as a piston in general discussions and to use the term in its broadest sense in order to include a displacer within its meaning. Thus the term piston includes all sliding bodies moving within a cylindrical vessel whether or not they experience pressure differentials on their surfaces while the term displacer is reserved for sliding bodies which experience essentially no pressure differential on their surfaces.

The apparatus of FIG. 1 is a two-staged refrigerator which operates on the so-called no-work cycle of U.S. Pat. No. 2,966,035. This refrigerator has a stepped fluid-tight housing 10 formed of a larger diameter section 11 and a smaller diameter section 12 joined through annular shoulder 13. Within housing section 11 the larger diameter displacer l4 defines in its motion a variable-volume warm chambers l5-and a first variable-volume refrigeration chamber 16. Within housing section 12 the smaller diameter displacer 17 defines a second, lower-temperature refrigeration chamber 18. Displacer 14 is mechanically linked to a driving means 19 through driving rod 20 and to the smaller diameter displacer 17 through a variable linkage 25 shown in somewhat simplified form to comprise an annular flange 26 on the upper or warmer end of displacer l7 and a flange-engageable ring in the form of a lip 27 extending inwardly within an open cavity 28 located in the bottom or colder end of displacer l4 and sized to permit the upper flanged end of displacer 17 to reciprocate therein with a small clearance. Exemplary constructions of this variable linkage and modifications thereon are shown in detail in FIGS. 4-7 and described below. Suitable seals 21 and 22 (either of the active or passive type) are provided for the two displacers l4 and 17, respectively, and if required, some form of shock-absorbing means such as a leather washer 23 may be affixed to the colder end of displacer 17 or to the internal end wall of housing section 12.

Displacer 14 has a regenerator 30, and displacer 17 a regenerator 31. The basic fluid flow path within the refrigerator comprises warm chamber 15, one or more fluid conduits 32 providing fluid communication between chamber and regenerator 30, the clearances around flange 26 and lip 27 connecting regenerator 30 with the first refrigeration chamber 16, regenerator 31 which is in direct fluid communication through cavity 28 with regenerator 30, one or more fluid conduits 33 providing fluid communication between regenerator 31 and the second colder refrigeration chamber 18 and refrigeration chamber 18.

High-pressure fluid is supplied by a high-pressure fluid source, e.g., the discharge side of compressor 34 through line 35 and valve 36 and low-pressure fluid is exhausted into a low pressure reservoir, e.g., the inlet side of compressor 34 through line 37 and valve 38. This fluid loop of a typical closed cycle also has a suitable fluid clean-up system shown at 39. Alternatively, in an open cycle the high-pressure supply 40 (e.g., a pressurized gas source) need not be connected with the low-pressure reservoir 41 (e.g., the atmosphere). FIG. 1 illustrates a mechanical driving means. It is also within the scope of this invention to use pneumatic driving means such as shown, for example, in US. Pat. Nos. 3,045,436 and 3,188,819.

Refrigeration is delivered at two temperaturelevels at a higher temperature level (typically 30 to 120K) from the fluid in refrigeration chamber 16 through a heat station 42 and at a lower temperature level (typically 8 to 25K) from the fluid in refrigeration chamber 18 through a heat station 43.

The apparatus of FIG. 2, wherein like reference nu merals are used to identify like components in FIG. 1, exemplifies a modification of the apparatus of FIG. 1 in that the regenerators 48 and 49 are located externally of the housing 10 and the fluid flow path comprises, in addition to regenerators 48 and 49, fluid conduits 50-53. The displacers 54 and 55, shown in cross hatching for convenience of identification, may be solid or hollow.

The three-staged refrigerator of FIG. 3 is constructed to have Although housing 60 formed of three different diameter housing sections 61, 62 and 63 and to have three displacers 64, 65 and 66 operable therein and sealed with seals 67, 68 and 69. Regenerators 70, 71 and 72 are located within their respective displacers. Variable linkage 73 provides the driving means for displacer 65 and linkage 74 for displacer 66. The displacers in their motion define variable volume refrigeration chambers 75, 76 and 77 which have associated therewith heat stations 78, 79 and 80, respectively. The flow path within the refrigerator is identical with that of FIG. 1 with an additional regenerator and refrigerator chamber and fluid conduit 59. ALthough the threestage refrigerator of FIG. 3 is shown to have intermediate-diameter displacer 64 connected to the displacer 65 and smallest-diameter displacer 66 connected to intermediate-diameter displacer 65 through variable linkage means, it is also within the scope of this invention to use only one variable linkage in a refrigerator having three or more stages, the variable linkage being employed between any two of the displacers.

The refrigerator of FIG. 3 is also shown to have a Joule-Thomson loop making it possible, if desired, to liquefy a gas. In keeping with known apparatus design, this Joule-Thomson loop comprises three heat exchangers 81, 82 and 83 adapted to effect refrigeration of the high-pressure fluid in the loop through out-ofcontact heat exchange between high-pressure fluid in line 84 and returning low-pressure fluid in line 85. The high-pressure fluid is also refrigerated at appropriate temperature levels by out-of-contact heat exchange with the fluid within the refrigeration chambers through heat stations 78, 79 and in heat exchangers 86, 87 and 88. A final Joule-Thomson heat exchanger 89 and expansion valve 90 are provided. Liquefied gas is collected in vessel 91 from which the low-pressure gas is returned through the loop in line 85.

FIGS. 4-7 illustrate three exemplary embodiments of the variable linkage, e.g., linkage 25 of FIGS. 1 and 2 and linkages 73 and 74 of FIG. 3. The refrigerator of FIG. 1 will be used as exemplary and the same reference numerals will be used in FIGS. 4-7 to identify the same apparatus components.

In the embodiment of the linkage shown in FIG. 4 the flow path provides for direct connection between the two regenerators 30 and 31. The flange 26 for the upper end of the smaller diameter displacer 17 is formed as a threaded cap adapted to engage threads in the internal wall 101 of displacer l7. Cap 100 has a central opening 102 providing a fluid port into regenerator 31 which is shown to comprise stacked screens 103. The engageable lip 27 is conveniently provided as a part of a threaded cap 104 adapted to engage threads in the internal wall 105 of displacer 14. When cap 104 is screwed in place, its outer flange 106 rests flush with a lower edge of displacer outer wall 107 to form a smooth end wall. Regenerator 31 terminates in a perforated disk 108 which engages the upper edge of cap 104 and which serves to retain the lead balls, screens or otherfilling providing the regenerative material in place within the regenerator. The under side of perforated disk 108 provides a contacting surface 109 which makes physical contact with the upper surface of lip 26 to drive displacer 17 downwardly. During upward movement the engagement is, of course, between flange 26 and lip 27.

It may be desirable to be able to vary the extent to which the stroke lengths of the two displacers differ, i.e., to control the length of travel of displacer 14 before engagement with displacer 17. This may conveniently be done by inserting an optional washer 99 into cap 104 to effectively raise the height of lip 27. It is, of course, within the scope of this invention to replace flange 26 with a series of two or more equally spaced pins extending radially outwardly from the top of cap 100 or to use any other suitable engaging means capable of linking the smallest-diameter piston to the largest-diameter piston as the latter experiences motion in a direction to effect an increase in volume of its associated refrigeration chamber.

In the embodment of FIG. 5, the variable linkage is modified to accomodate a fluid flow path of the basic type described in U.S. Pat. No. 3,218,815. The end cap 110 of displacer 17 is solid and fluid ports 111 through the displacer wall are spaced at intervals to provide a fluid passage into regenerator 31. Displacer 14 is configured to have a lower section 112 of somewhat smaller diameter than the remaining part of the displacer thereby to define an annular fluid flow passage 113 which opens into a number of radial passages, two of which are shown at 114, which in turn lead into regenerator 31 which terminates in an impervious bottom wall 115. The surface 116 of cavity 28 serves as the contacting face for the engagement of the displacer during downward movement. The flow of high-pressure fluid is shown by arrows. It is, of course, reversed for the returning low-pressure fluid. The fluid flow path of the embodiment of FIG. offers thermal advantages, but these may in part be offset by a somewhat higher pressure drop because of the indirect flow path.

FIG. 6 shows a modification of the variable linkage of FIG. 4 wherein an engaging post 1 17 is affixed to the perforated disk 108 of displacer 14 to provide a flange 118 which is sized to engage an inwardly extending lip 119 around the opening of a cavity 28a in the engaging end of displacer 17. A foraminous plate 108a is shown as a cover for regenerator 31. It will be apparent that the operation of the apparatus of FIG. 6 is identical to that of the apparatus of FIG. 4.

The variable linkages of FIGS. 4-6 rely upon seal friction against pressure drop to keep the smaller diameter displacers in place until engaged. However, it may be desirable in some cases to provide positive means for ensuring that displacer 17 does not experience unwanted motion. Such means are provided in the form ofa spring 1 16 in compression as shown in FIG. 7. The effect of the use of such a spring will be described in the discussion of FIGS. 8-15 which follows.

The use of the variable linkage in the stepped, staged refrigerator of this invention has important thermodynamic significance which can best be explained with reference to FIGS. 8-I5. FIGS. 8-11 detail the operation of the linkage of the apparatus of this invention as it is illustrated in the embodiment of FIG. 1 using the linkage detailed in FIGS. 4-6. No attempt has been made to detail the apparatus components in these FIGS. 8-11 and the largerand smaller-diameter displacers are cross hatched for ease of identification.

It may be assumed in this description of the operation of this apparatus that a cycle begins with the complete exhaustion of essentially all of the low-pressure cold fluid (except that in void volumes) from all of the refrigeration chambers, i.e., chambers 16 and 18. This necessitates the closing of low-pressure valve 38 and the opening of high-pressure valve 36. With the introduction of high-pressure fluid into the system, the upper, larger-diameter displacer is moved upwardly (FIG. 9) thus defining a first refrigeration chamber 16 of increasing volume while displacer 17 remains in its lowermost position by virtue of the seal friction of seal, 22 (FIG. 1), After displacer 14 has experienced a predetermined portion of its total stroke length, the variable linkage causes displacer 14 to engage displacer 17 through flange 26 and lip 27 and to raise it. In a preferred embodiment of this cycle, the high-pressure fluid is cut off before refrigeration chambers 16 and .18 attain their full volume (early cutoff). As the largerdiameter displacer 14 continues to be moved upwardly the volume of refrigeration chamber 18 increases. At the end of this upward movement the stroke length 'of displacer 14 is 1 while that of displacer 17 is I and so the ratio. of the final volumes of refrigeration cham: bers 16 and 18 is a function of 1 /1 as well as of the diameters of displacers l4 and 17.

With the attainment of maximum volume (FIG. 10) the exhaust valve is opened. Seal friction is sufficient to hold displacer 17 inthe position shown in FIG. 10 even though the highpressure gas exhausting from chamber 18 through regenerator 31 results in a net force on displacer 17 in the upward direction. Displacer 14 is moved downwardly until contacting surface 109 engages flange 26 of the smaller diameter displacer (FIG. 11). This downward motion of both displacers then effects the final exhaustion of the fluid from the refrigerator to return it to the condition required to begin another cycle (FIG. 8).

The effect of the variable linkage on the PV diagrams for the refrigerator is clearly seen in FIGS. 13 and 14. FIG. 13 is a PV diagram for the first or larger diameter refrigeration chamber l6and it follows a typical PV curve for the cycle with early cut-off. (Volumes in these PV diagrams are expressed in terms of aribtrary units of stroke length of the displacer which defines the volume.) By restricting the motion and stroke length of displacer 17 the lower-temperature refrigeration chamber 18 has the PV diagram of FIG. 14 which is essentially the right-hand portion of the PV diagram of FIG. 13. One result of the attainment of these two different PV diagrams for the two refrigeration chambers is the realization of a fuller PV diagram for the larger volume refrigeration chamber which in turn means that the fluid in this chamber is colder by several degrees K. This in turn, coupled with the attainment of a larger displacement ratio (e.g., ratio of the volume of the larger-diameter chamber to the volume of the smallerdiameter chamber) results in decreased regenerator losses; for the colder the fluid is in the first larger refrigerator chamber the smaller is the AT across the regenerator. This in turn means that the fluid in the refrigeration chamber at the lowest temperature level reaches a lower temperature or is capable of delivering more refrigeration at a fixed temperature. Moreover, the regenerator associated with the colder refrigeration chamber (e.g., regenerator 31 of FIG. 1) is normally formed of a mass of lead balls since lead has a high heat capacity at these low temperatures. However, lead has a relatively low specific heat, Cp, at these temperatures. By reducing the mass flow through the lead (brought about by decreasing the temperature in the larger-diameter refrigeration chamber and curtailing the volume of the smaller diameter refrigeration chamber) the lead experiences a smaller temperature swing, AT, and thereby becomes a more efficient heat transfer means. I

The use of spring 116 in the embodiment of FIG. 7 alters the end of the stroke as shown in FIG. 12 which is equivalent to the same point in the cycle illustrated in FIG. 11 for a variable linkage without the positive holding action of the spring. Itwill be seen that the spring force tends to hold displacer 17 in spaced relation with displacer I4, and as a result displacer 17 is moved downwardly at the time displacer 14 starts its downward motion. This in turn gives rise to a somewhat modified PV diagram for chamber 18 as illustrated in FIG. 15. However, the overall results of colder temperatures, decreased mass flow into refrigeration chamber 18 and decreased regenerator losses are essentially the same.

FIG. 16 illustrates an embodiment of the apparatus of this invention wherein the variable linkage between the displacers is a combination of surface engagement between the displacers and actuating/restraining means associated with the smaller diameter displacer only. The lower portion of the refrigerator illustrated is of the same two-staged type of FIG. 1 and like reference numerals are used to refer to like components. In the apparatus embodiment of this invention of FIG. 16, the larger-diameter displacer 120 (having regenerator 121 and fluid conduits 122) is not linked to the smaller diameter displacer 123 (having regenerator 124) by any mechanical linkage; but displacer 120 does impart downward motion to displacer 124 when the bottom driving surface 125 of displacer 120 makes contact with the upper surface 126 of the perforated disk 127 serving as the ported cover for regenerator 124.

Because of the lack of any separate mechanical linkage between displacers 120 and 123, some means must be provided to control the upward motion of displacer 123 and adjust its stroke length. This means is provided in the form shown in detail in FIG. 16. A post 130, having an engaging flange 131, is affixed such as by screw 132, to the bottom 133 of housing 12, and the colder end of displacer 123 is modified to provide a flangeengaging lip 134 extending inwardly from the mouth of cavity 135 which is sized to permit a predetermined length of stroke of displacer 123 before lip 134 engages flange 131. A spring 136 in compression maintains displacer 123 in its upwardmost position during those periods in the cycle when displacer 120 is not exerting a downward force on displacer 123. The fluid flow path between regenerator 124 and refrigeration chamber 18 is essentially that of US. Pat. No. 3,218,815 and includes radial passage 137 and annular passage 138.

The operation of the embodiment of FIG. 16 is illustrated in FIGS. 17-20. From FIGS. 17 it will be seen that the same conditions apply at startup as were noted in FIG. 8 for the apparatus embodiment of FIG. 1. However, when high-pressure fluid begins to be introduced into the refrigerator a portion thereof immediately enters chamber 18, the volume of which begins to increase immediately and is finally limited by the length of post 130. Chamber 16 continues to experience increasing volume after maximum volume is attained in chamber 18. This is different from the embodiment of FIG. 1 as will be clear from a comparison between FIGS. 18 and 19 with FIGS. 9 and 10. However, the downward strokes of the two apparatus are identical in their results. FIG. 20, which is a PV diagram for the colder refrigeration chamber 18 of the refrigerator of FIG. 16, shows that it is essentially equivalent to the left-hand side of the PV diagram for the largerdiameter refrigeration chamber as shown in FIG. 13.

Apparatus may be constructed according to this invention to operate on the cycle of US. Pat. No. 2,906,101 as shown in FIG. 21. The apparatus of FIG. 21 differs from that of FIG. 1 in that the warm chamber is not present. In accordance with the difference of the two cycles involved, the cycle of FIG. 21 delivers some external mechanical energy in place of the thermal energy delivered by the refrigerator of FIG. 1 operating on the no-w0rk cycle.

In FIG. 21 like reference numerals are used to refer to like elements shown in FIG. 1. As indicated above, fluid-tight stepped housing 10 is open at the top and the upper section 140 is preferably constructed to have a thicker wall than the remaining portion of housing 10. Some modification is required in section 140 of the housing inasmuch as the high-pressure fluid must be introduced directly into and low-pressure fluid exhausted directly from regenerator 30. This modification in FIG. 21 takes the form of a multiplicity of slots 141 in wall 140. High-pressure fluid is brought into the system and low-pressure fluid exhausted from it through line 142 which is, of course, equivalent to line 35 and- 37 of FIG. 1. This external conduit is in fluid communication with the regenerator through slots 141, an outer annular passage 143, an annular passage 144 out into the side of the displacer wall and a series of radial passages 145 leading into regenerator 30. The fluid flow path between chambers 16 and 18 by way of regenerators 30 and 31 may be identical to that described above for FIGS. 4 and 5. Sealing rings 146 and 147 are provided so that displacer 14 may make fluid-tight contact with the inner wall housing section 140. v

The advantages realized in the apparatus of FIG. 21 are essentially the same as those described for the apparatus of FIG. 1. The modifications in fluid flow path, the use of a spring as shown in FIG. 7 and the embodiment of the variable linkage illustrated in FIG. 16 are all applicable to the apparatus of FIG. 21.

FIG. 22 illustrates the application of this invention to one embodiment of a Stirling engine which may briefly be described as an apparatus which includes an additional compressor piston operable within the housing to define a compressor chamber of variable volume to alternately serve as a source of high-pressure fluid and as a low-pressure fluid reservoir. In the embodiment of the Stirling engine illustrated in FIG. 22 the expansionrefrigeration portion of the housing 150 is of a stepped configuration separated from the compressor housing 151. However, the compressor chamber may be located in the same housing as the expansion chamber and the housings may be considered as a single housing means. In FIG. 22 the two housing parts are joined by a suitable fluid conduit 152 having a aftercooler 153 incorporated in it. Conduit means 154 is provided to circulate a cooling fluid through the aftercooler. Compression of the fluid within the engine takes place in chamber 156 and is effected by means of a piston 157 driven by rod 158 from a suitable driving means 159. In keeping with this invention the expansion is accomplished in two expansion chambers 160 and 161 through the movement of a larger-diameter piston 162 mechanically linked through rod 163 with driving means 159 and a smaller-diameter piston 164 operable within stepped housing 150 formed of a larger diameter section 165 and a smaller-diameter section 166. Pistons 162 and 164 are variably linked as described in detail in FIGS. 4-6 and regenerators 167 and 168 are located internally of pistons 162 and 164, respectively.

Within housing 150 large diameter piston 162 moves to define refrigeration chamber 160 and an intermediate-temperature chamber 170. The flow path within the expansion portion of the apparatus will be seen to be comprised of a passage 173 drilled in the header 174, intermediate-temperature chamber 170,

passage 175, regenerator 167, the first larger volume expansion chamber 160, regenerator 168, and the second smaller volume refrigeration chamber 161. Inasmuch as the Stirling cycle is well known, it will not be further detailed here. Since the staged Stirling cycle involves the delivery of refrigeration at two temperature levels, the use of regenerators, and considerations of mass flow similar to those of the cycle on which the apparatus of FIG. 1 operates, it follows that essentially the same advantages of lower temperatures and/or greater refrigeration capacity are to be gained by adjusting the refrigeration chamber volumes by the combination of diameter and stroke length control.

FIG. 23 illustrates the application of this invention to a refrigeration apparatus designed to operate on the Vuilleumier cycle. The operation of this cycle requires that there be provided a hot chamber and a chamber of intermediate-temperature and one or more cold expansion chambers, these terms being relative for any one system. Thus the basic apparatus for carrying out the Vuilleumier cycle includes, in addition to one or more displacers for defining variable-volume refrigeration chambers, additional displacer means to define a high-temperature variable-volume chamber and an intermediate-temperature variable-volume chamber which in combination serve as a source of highpressure fluid and a reservoir for low-pressure fluid. The Vuilleumier cycle is a completely closed cycle and the flow path which joins these chambers includes heat storage means such as regenerators. In the apparatus of FIG. 23 the expansion chambers along with a portion of the intermediate-temperature chamber are found in stepped housing 180 formed of a larger-diameter section 181 and a smaller diameter section 182; and the hot chamber along with the remaining portion of the intermediate-temperature is found in a housing 183. Within housing 183 is located a warm (or hot) chamber 184. The intermediate-tomperature chamber is made up of a combination of chambers 185 located within housing 183 and chamber 186 located in housing 180. These are joined through a conduit 187 which has associated with it a heat exchanger 188. Conduit 187 passes down through the header 189 which serves as one end of housing 180. Within housing 183 is a piston 190 containing a regenerator 191. It is, of course, within the scope of this invention and in the operation of the cycle to locate the regenerator external of the housing if this is desired. Piston 190 is driven by means of a suitable driving means such as a crank 192 to which the piston rod 193 is mechanically linked. Inasmuch as the operation of this cycle requires that heat (thermal energy for compression) be introduced into the system in the fluid in the hot chamber 184, there is provided coils 194 which are in thermal contact with the outer wall of housing 783 and through which a heating fluid may be passed. Alternatively, if the temperature of the refrigerator is such that the heat put into the fluid in chamber 184 is at room temperature, the coils may be replaced with heat transfer fins.

Within stepped housing 180, there are defined two expansion chambers, the first 195 which is at an intermediate low-temperature and the second 196 which is at the coldest temperature of the refrigerator. These are defined through the movement of larger-diameter pistons 197 and smaller-diameter piston 198 which are joined through a variable linkage 199 such as that de tailed in FIGS. 4-7. Suitable sealing means, shown as O-ring seals 200 and 201, are provided to isolate the refrigeration chambers.

Displacers 197 isdriven by a piston rod 202 which is also attached to crank 192. Generally, rods 193 and 202 will be about degrees out of phase; however, this may be varied and may easily be determined for any set of conditions by one skilled in the art.

The fluid flow path within the apparatus will be seen to comprise (reading from chamber 184 to chamber 196) the passages 203, regenerator 191, passages 204, chamber 185, conduit 187, chamber 186, passage 215, regenerator 205 in piston 197, chamber 195, regenerator 206, in displacer 198, and chamber 196.

Inasmuchas the cycle itself is not part of this invention, it need not be described in detail. A complete description of the cycle on which this apparatus operates can be found in US. Pat. No. l,275,507. It will be apparent that the apparatus of FIG. 23 realizes the same operational advantages as that of FIGS. 1, 20 and 21.

It will, of course, be apparent to those skilled in the art that the apparatus of FIGS. 21-23 may be constructed to have more than two refrigeration stages and that the pistons associated with one or more stages may be joined by variable linkages as described for the apparatus of FIG. 3. Moreover, such modifications as the fluid flow path of FIG. 5, the spring of FIG. 1, the linking means of FIG. 16 and pneumatic driving means may be applied to the apparatus of FIGS. 21-23.

The apparatus of this invention therefore provides a means whereby the volumes of those refrigeration chambers in a multistaged cryogenic refrigerator which are maintained at the lower levels may be adjusted by using an optimum combination of chamber diameter and the length of the stroke of the piston defining the chambers in a housing.

lt'will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. In a temperature-staged cryogenic apparatus wherein at least two directly connectable pistons of different diameters reciprocate within a fluid-tight housing of steppedconfiguration to define at least two refrigeration chambers of different diameters capable of delivering refrigeration at different temperature levels, wherein said refrigeration chambers are connected by a fluid flow path and wherein the largest-diameter piston is connected to external driving means, the improvement comprising variable piston linkage means adapted to provide engagement between said pistons and arranged to cause said largest-diameter piston to impart reciprocal motion to at least one of the other of said pistons through a stroke length less than that experienced by said largest diameter piston.

2. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of one of said pistons; and

b. lip means, adapted to engage said flange means,

affixed to the engaging end of another of said pistons.

3. A cryogenic apparatus in accordance with claim 2 wherein said flange means are affixed to said largestdiameter piston and said lip means to said other of said pistons.

4. A cryogenic apparatus in accordance with claim 2 wherein said lip means are affixed to said largestdiameter piston and said flange means to said other of said pistons.

5. A cryogenic apparatus in accordance with claim 2 including spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of the largest-diameter refrigeration chamber is increasing and while it remains at maximum.

6. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of said other of said pistons; and

b. a cavity within said largest-diameter piston providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having inwardly directed lip means adapted to enge said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged.

7. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. lip means terminating the engaging end of said other of said pistons;

b. a cavity within said largest-diameter piston providing a contacting surface, having an opening into the largest-diameter refrigeration chamber and being sized to permit therein the motion of said other of said pistons engaged; and

c. flanged post means adapted to engage said lip means, affixed to said contacting surface and extending into said cavity.

8. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. post means affixed to that portion of the wall of said housing which defines the end of the smallestdiameter refrigeration chamber, said post means terminating in flange means;

b. a cavity within said other of said pistons which in its motion defines said smallest-diameter refrigeration chamber, said cavity providing an engagement surface and opening into said smallest-diameter refrigeration chamber to provide an inwardly directed lip means adapted to engage said flange; and

spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said smallest diameter refrigeration chamber is increasing and while it remains at maximum. 9. in a temperature-staged cryogenic apparatus wherein at least two directly connectable pistons of different diameters reciprocate within a fluid-tight housing of stepped configuration to define at least two refrigeration chambers of different diameters capable of delivering refrigeration at different temperature levels, wherein said refrigeration chambers are connected by a fluid flow path which includes heat storage means located within said pistons and wherein the largestdiameter piston is connected to external driving means, the improvement comprising variable piston linkage means adapted to provide engagement between said pistons and arranged to cause said largest-diameter piston to impart reciprocal motion to at least one of the other of said pistons through a stroke length less than that experienced by said largest-diameter piston.

10. A cryogenic apparatus in accordance with claim 9 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of one of said pistons; and

b. lip means, adapted to engage said flange means,

affixed to the engaging end of another of said pistons.

11. A cryogenic apparatus in accordance with claim 10 wherein said flange means are affixed to said largestdiameter piston and said lip means to said other of said pistons.

12. A cryogenic apparatus in accordance with claim 10 wherein said lip means are affixed to said largestdiameter piston and said flange means to said other of said pistons.

13. A cryogenic apparatus in accordance with claim 9 wherein said variable piston linkage means comprise a. first cap means affixed to the engaging end of said other of said pistons, said cap means terminating in a flange and being arranged to provide fluid communication into the heat storage means located within said other of said pistons; and

b. second cap means affixed to said largest-diameter piston, said second cap means providing a contacting surface and having an opening into the largestdiameter refrigeration chamber, said opening having inwardly directed lip means adapted to engage said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged and providing fluid communication means between said cavity and the heat storage means located within said largest-diameter piston.

14. A cryogenic apparatus in accordance with claim 13 including spring means adapted to maintain said flange in engagement with said lip means. during those times in the operation of said apparatus when the volume of said largest-diameter refrigeration chamber is increasing and while it remains at maximum.

15. A temperature-staged cryogenic apparatus for delivering refrigeration at at least two temperature levels, comprising in combination a. housing means formed of at least two cylindrical sections of different diameters to provide a fluidtight enclosure of stepped configuration;

b. at least two directly connectable pistons of different diameters adapted to move within an associated section of said housing means and in their motion to define at least two variable-volume refrigeration chambers of different diameter and different maximum volume, the temperatures attainable in said chambers decreasing with decreasing chamber diameter;

c. fluid flow path means providing fluid communication between said refrigeration chambers and incorporating heat storage means;

(1. external driving means associated with the largest- I diameter piston and adapted to control its motion within said housing; and

e. variable piston linking means providing engagement between said pistons and arranged to cause said largest-diameter piston to impart motion to the other of said pistons, at least one of which is caused to experience a stroke length less than that experi: enced by said largest-diameter piston.

16. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linkage means comprise a. flange means aflixed to the engaging end of one of said pistons; and l b. lip means, adapted to engage said flange means,

afiixed to the engaging end of another of said pistons.

17. A cryogenic apparatus in accordance with claim 16 wherein said flange means are affixed to said largestdiameter piston and said lip means to said other of said pistons.

18. A cryogenic apparatus in accordance with claim 16 wherein said lip means are affixed to said largestdiameter piston and said flange means to said other of said pistons.

19. A cryogenic apparatus in accordance with claim 16 including spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said largest-diameter refrigeration chamber is increasing and while it remains at maximum.

20. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linking means comprise a. flange means affixed to the engaging end of said other of said pistons; and b. a cavity within said largest-diameter piston providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having lip means adapted to engage said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged. 21. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linkage means comprise a. post means affixed to that portion of the wall of said housing which defines the end of the smallestdiameter refrigeration chamber, said post means terminating in flange means;

b. a cavity within said other of said pistons which in its motion defines said smailest-diameter refrigeration chamber said cavity providing an engagement surface and opening into said smallest-diameter refrigeration chamber to provide an inwardly directed lip means adapted to engage said flange; and

c. spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said smallest-diameter refrigeration chamber is increasing and while it remains at maximum.

22. An apparatus in accordance with claim 15 wherein said heat storage means are located within said pistons and said variable piston linkage means comprise a. first cap means affixed to the engaging end of said other of said pistons, said cap means terminating in a flange and being arranged to provide fluid communication means into the heat storage means located within said other of said pistons; and

23. An apparatus in accordance with claim 15 I wherein said largest-diameter piston in its motion also defines an additional chamber of variable volume which is in fluid communication through said fluid flow path with said refrigeration chambers.

24. An apparatus in accordance with claim 15 including a. a high-pressure fluid source; and

b. a low-pressure fluid reservoir.

25. An apparatus in accordance with claim 24 wherein said high-pressure fluid source and said lowpressure fluid reservoir are located externally of said housing and wherein said apparatus includes valved conduit means providing fluid communication between said high-pressure fluid source and said low-pressure fluid reservoir and said refrigeration chambers.

26. An apparatus in accordance with claim 25 wherein said high-pressure fluid source and said lowpressure fluid reservoir are combined in a compressor.

27. An apparatus in accordance with claim 24 including within said housing means an additional piston operable to define a variable-volume compression chamber adapted to serve as said high-pressure fluid source and as said low-pressure fluid reservoir.

28. An apparatus in accordance with claim 24 including within said housing means displacer means defining high-temperature and intermediate-temperature chambers of variable volumes in fluid communication through heat storage means, said high-temperature and intermediate-temperature chambers in combination being adapted to serve as said high-pressure fluid source and said low-pressure fluid reservoir.

29. An apparatus in accordance with claim 15 wherein said housing is formed of three cylindrical sections and three pistons move within said housing, at least one of the other of said pistons being caused to experience a stroke length less than that experienced by said largest-diameter piston.

30. An apparatus in accordance with claim 15 wherein said housing is formed of three cylindrical sections and wherein the piston moving within the intermediate-diameter section of said housing is linked through said variable piston linking means to said largest-diameter piston and the piston moving within the smallest-diameter section of said housing is linked through said variable piston linking means to said intermediate-diameter piston, whereby the length of the stroke of said intermediate-diameter piston is less than that of said largest-diameter piston and the length of the stroke of said smallest-diameter piston is less than that of said intermediate diameter piston.

31. An apparatus in accordance with claim 29 including a. an external high-pressure fluid source;

b. an external low-pressure fluid reservoir; and

c. valve-controlled fluid conduit means arranged to provide fluid communication between said chambers and said external high-pressure fluid source and said low-pressure fluid reservoir.

32. An apparatus in accordance with claim including stroke adjusting means associated with said variable piston linkage means adapted to control the length of the stroke of said other of said pistons.

33. A temperature-staged cryogenic apparatus adapted to liquefy a gas comprising in combination a. housing means formed of three cylindrical sections of different diameters to provide a fluid-tight enclosure of stepped configuration;

b. three directly connectable pistons of different diameters adapted to move within associated sections of said housing means and in their motion to define three variable-volume refrigeration chambers of different diameter and different maximum volume, the temperatures attainable in said chambers decreasing with decreasing chamber diameter;

c. fluid flow path means providing fluid communication between said refrigeration chambers and incorporating heat storage means;

(1. a first high-pressure fluid source;

e. a first low-pressure fluid reservoir;

f. external driving means associated with the largestdiameter piston and adapted to control its motion within said housing; g. variable piston linking means providing engagement between said pistons and arranged to cause said largest-diameter piston to impart motion to the other two of said pistons, at least one of which is caused to experience a stroke length less than that experienced by said largest-diameter piston;

h. heat station means associated with each of said refrigeration chambers and being adapted to deliver refrigeration through out-of-contact heat exchange with fluid in said refrigeration chambers at three decreasing temperature levels;

i. a second high-pressure fluid source external of said housing means;

j. a second low-pressure fluid reservoir external of said housing means;

k. liquefaction means adapted to expand and at least partially liquefy high-pressure fluid from said second high-pressure fluid source;

1. vessel means adapted to receive the liquefied fluid and to provide a source of cold low-pressure fluid;

m. heat exchange means incorporating said heat station means at said three temperature levels adapted to effect out-of-contact heat exchange between fluid from said second high-pressure fluid source delivered to said liquefaction means and said cold low-pressure fluid delivered from said vessel means to said second low-pressure fluid reservoir.

34. An apparatus in accordance with claim 33 wherein said first and second high-pressure fluid sources are combined as one and said first and second low-pressure fluid reservoirs are combined as one, and further including valve-controlled fluid conduit means adapted to provide fluid communication between the combined high-pressure fluid source and the combined low-pressure fluid reservoir and said fluid flow path means.

35. An apparatus in accordance with claim 34 wherein said low-pressure fluid reservoir and said highpressure fluid source are combined in a compressor.

36. An apparatus in accordance with claim 33 including within said housing means an additional piston operable to define a variable-volume compression chamber adapted to serve as said first high-pressure fluid source and as said first low-pressure fluid reservoir.

37. An apparatus in accordance with claim 33 including within said housing means displacer means defining high-temperature and intermediate-temperature chambers of variable volumes in fluid communication through heat storage means, said high-temperature and intermediate-temperatures in combination being adapted to serve as said first high-pressure fluid source and said first low-pressure fluid reservoir. 

1. In a temperature-staged cryogenic apparatus wherein at least two directly connectable pistons of different diameters reciprocate within a fluid-tight housing of stepped configuration to define at least two refrigeration chambers of different diameters capable of delivering refrigeration at different temperature levels, wherein said refrigeration chambers are connected by a fluid flow path and wherein the largest-diameter piston is connected to external driving means, the improvement comprising variable piston linkage means adapted to provide engagement between said pistons and arranged to cause said largest-diameter piston to impart reciprocal motion to at least one of the other of said pistons through a stroke length less than that experienced by said largest diameter piston.
 2. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of one of said pistons; and b. lip means, adapted to engage said flange means, affixed to the engaging end of another of said pistons.
 3. A cryogenic apparatus in accordance with claim 2 wherein said flange means are affixed to said largest-diameter piston and said lip means to said other of said pistons.
 4. A cryogenic apparatus in accordance with claim 2 wherein said lip means are affixed to said largest-diameter piston and said flange means to said other of said pistons.
 5. A cryogenic apparatus in accordance with claim 2 inclUding spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of the largest-diameter refrigeration chamber is increasing and while it remains at maximum.
 6. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of said other of said pistons; and b. a cavity within said largest-diameter piston providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having inwardly directed lip means adapted to enge said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged.
 7. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. lip means terminating the engaging end of said other of said pistons; b. a cavity within said largest-diameter piston providing a contacting surface, having an opening into the largest-diameter refrigeration chamber and being sized to permit therein the motion of said other of said pistons engaged; and c. flanged post means adapted to engage said lip means, affixed to said contacting surface and extending into said cavity.
 8. A cryogenic apparatus in accordance with claim 1 wherein said variable piston linkage means comprise a. post means affixed to that portion of the wall of said housing which defines the end of the smallest-diameter refrigeration chamber, said post means terminating in flange means; b. a cavity within said other of said pistons which in its motion defines said smallest-diameter refrigeration chamber, said cavity providing an engagement surface and opening into said smallest-diameter refrigeration chamber to provide an inwardly directed lip means adapted to engage said flange; and c. spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said smallest diameter refrigeration chamber is increasing and while it remains at maximum.
 9. In a temperature-staged cryogenic apparatus wherein at least two directly connectable pistons of different diameters reciprocate within a fluid-tight housing of stepped configuration to define at least two refrigeration chambers of different diameters capable of delivering refrigeration at different temperature levels, wherein said refrigeration chambers are connected by a fluid flow path which includes heat storage means located within said pistons and wherein the largest-diameter piston is connected to external driving means, the improvement comprising variable piston linkage means adapted to provide engagement between said pistons and arranged to cause said largest-diameter piston to impart reciprocal motion to at least one of the other of said pistons through a stroke length less than that experienced by said largest-diameter piston.
 10. A cryogenic apparatus in accordance with claim 9 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of one of said pistons; and b. lip means, adapted to engage said flange means, affixed to the engaging end of another of said pistons.
 11. A cryogenic apparatus in accordance with claim 10 wherein said flange means are affixed to said largest-diameter piston and said lip means to said other of said pistons.
 12. A cryogenic apparatus in accordance with claim 10 wherein said lip means are affixed to said largest-diameter piston and said flange means to said other of said pistons.
 13. A cryogenic apparatus in accordance with claim 9 wherein said variable piston linkage means comprise a. first cap means affixed to the engaging end of said other of said pistons, said cap means terminating in a flange and being arranged to provide fluid communication into the heat storage means located within said other of said pistons; and b. second cap means affixed to said largest-diameter piston, said second cap means providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having inwardly directed lip means adapted to engage said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged and providing fluid communication means between said cavity and the heat storage means located within said largest-diameter piston.
 14. A cryogenic apparatus in accordance with claim 13 including spring means adapted to maintain said flange in engagement with said lip means during those times in the operation of said apparatus when the volume of said largest-diameter refrigeration chamber is increasing and while it remains at maximum.
 15. A temperature-staged cryogenic apparatus for delivering refrigeration at at least two temperature levels, comprising in combination a. housing means formed of at least two cylindrical sections of different diameters to provide a fluid-tight enclosure of stepped configuration; b. at least two directly connectable pistons of different diameters adapted to move within an associated section of said housing means and in their motion to define at least two variable-volume refrigeration chambers of different diameter and different maximum volume, the temperatures attainable in said chambers decreasing with decreasing chamber diameter; c. fluid flow path means providing fluid communication between said refrigeration chambers and incorporating heat storage means; d. external driving means associated with the largest-diameter piston and adapted to control its motion within said housing; and e. variable piston linking means providing engagement between said pistons and arranged to cause said largest-diameter piston to impart motion to the other of said pistons, at least one of which is caused to experience a stroke length less than that experienced by said largest-diameter piston.
 16. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linkage means comprise a. flange means affixed to the engaging end of one of said pistons; and b. lip means, adapted to engage said flange means, affixed to the engaging end of another of said pistons.
 17. A cryogenic apparatus in accordance with claim 16 wherein said flange means are affixed to said largest-diameter piston and said lip means to said other of said pistons.
 18. A cryogenic apparatus in accordance with claim 16 wherein said lip means are affixed to said largest-diameter piston and said flange means to said other of said pistons.
 19. A cryogenic apparatus in accordance with claim 16 including spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said largest-diameter refrigeration chamber is increasing and while it remains at maximum.
 20. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linking means comprise a. flange means affixed to the engaging end of said other of said pistons; and b. a cavity within said largest-diameter piston providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having lip means adapted to engage said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged.
 21. A cryogenic apparatus in accordance with claim 15 wherein said variable piston linkage means comprise a. post means affixed to that portion of the wall of said housing which defines the end of the smallest-diameter refrigeration chamber, said post means terminating in flange means; b. a cavity within said other of said pistons which in its motion defines said smallest-diameter refrigeration chamber said cavity providing an engagement surface and opening into said smallest-diameter refrigeration chamber to provide an inwardly directed lip means adapTed to engage said flange; and c. spring means adapted to maintain said flange means in engagement with said lip means during those times in the operation of said apparatus when the volume of said smallest-diameter refrigeration chamber is increasing and while it remains at maximum.
 22. An apparatus in accordance with claim 15 wherein said heat storage means are located within said pistons and said variable piston linkage means comprise a. first cap means affixed to the engaging end of said other of said pistons, said cap means terminating in a flange and being arranged to provide fluid communication means into the heat storage means located within said other of said pistons; and b. second cap means affixed to said largest-diameter piston, said second cap means providing a contacting surface and having an opening into the largest-diameter refrigeration chamber, said opening having inwardly directed lip means adapted to engage said flange, said cavity being sized to permit therein the motion of said other of said pistons engaged and providing fluid communication means between said cavity and the heat storage means located within largest-diameter.
 23. An apparatus in accordance with claim 15 wherein said largest-diameter piston in its motion also defines an additional chamber of variable volume which is in fluid communication through said fluid flow path with said refrigeration chambers.
 24. An apparatus in accordance with claim 15 including a. a high-pressure fluid source; and b. a low-pressure fluid reservoir.
 25. An apparatus in accordance with claim 24 wherein said high-pressure fluid source and said low-pressure fluid reservoir are located externally of said housing and wherein said apparatus includes valved conduit means providing fluid communication between said high-pressure fluid source and said low-pressure fluid reservoir and said refrigeration chambers.
 26. An apparatus in accordance with claim 25 wherein said high-pressure fluid source and said low-pressure fluid reservoir are combined in a compressor.
 27. An apparatus in accordance with claim 24 including within said housing means an additional piston operable to define a variable-volume compression chamber adapted to serve as said high-pressure fluid source and as said low-pressure fluid reservoir.
 28. An apparatus in accordance with claim 24 including within said housing means displacer means defining high-temperature and intermediate-temperature chambers of variable volumes in fluid communication through heat storage means, said high-temperature and intermediate-temperature chambers in combination being adapted to serve as said high-pressure fluid source and said low-pressure fluid reservoir.
 29. An apparatus in accordance with claim 15 wherein said housing is formed of three cylindrical sections and three pistons move within said housing, at least one of the other of said pistons being caused to experience a stroke length less than that experienced by said largest-diameter piston.
 30. An apparatus in accordance with claim 15 wherein said housing is formed of three cylindrical sections and wherein the piston moving within the intermediate-diameter section of said housing is linked through said variable piston linking means to said largest-diameter piston and the piston moving within the smallest-diameter section of said housing is linked through said variable piston linking means to said intermediate-diameter piston, whereby the length of the stroke of said intermediate-diameter piston is less than that of said largest-diameter piston and the length of the stroke of said smallest-diameter piston is less than that of said intermediate diameter piston.
 31. An apparatus in accordance with claim 29 including a. an external high-pressure fluid source; b. an external low-pressure fluid reservoir; and c. valve-controlled fluid conduit means arranged to provide fluid communication between said chambers and said external high-pressure fluid source anD said low-pressure fluid reservoir.
 32. An apparatus in accordance with claim 15 including stroke adjusting means associated with said variable piston linkage means adapted to control the length of the stroke of said other of said pistons.
 33. A temperature-staged cryogenic apparatus adapted to liquefy a gas comprising in combination a. housing means formed of three cylindrical sections of different diameters to provide a fluid-tight enclosure of stepped configuration; b. three directly connectable pistons of different diameters adapted to move within associated sections of said housing means and in their motion to define three variable-volume refrigeration chambers of different diameter and different maximum volume, the temperatures attainable in said chambers decreasing with decreasing chamber diameter; c. fluid flow path means providing fluid communication between said refrigeration chambers and incorporating heat storage means; d. a first high-pressure fluid source; e. a first low-pressure fluid reservoir; f. external driving means associated with the largest-diameter piston and adapted to control its motion within said housing; g. variable piston linking means providing engagement between said pistons and arranged to cause said largest-diameter piston to impart motion to the other two of said pistons, at least one of which is caused to experience a stroke length less than that experienced by said largest-diameter piston; h. heat station means associated with each of said refrigeration chambers and being adapted to deliver refrigeration through out-of-contact heat exchange with fluid in said refrigeration chambers at three decreasing temperature levels; i. a second high-pressure fluid source external of said housing means; j. a second low-pressure fluid reservoir external of said housing means; k. liquefaction means adapted to expand and at least partially liquefy high-pressure fluid from said second high-pressure fluid source; l. vessel means adapted to receive the liquefied fluid and to provide a source of cold low-pressure fluid; m. heat exchange means incorporating said heat station means at said three temperature levels adapted to effect out-of-contact heat exchange between fluid from said second high-pressure fluid source delivered to said liquefaction means and said cold low-pressure fluid delivered from said vessel means to said second low-pressure fluid reservoir.
 34. An apparatus in accordance with claim 33 wherein said first and second high-pressure fluid sources are combined as one and said first and second low-pressure fluid reservoirs are combined as one, and further including valve-controlled fluid conduit means adapted to provide fluid communication between the combined high-pressure fluid source and the combined low-pressure fluid reservoir and said fluid flow path means.
 35. An apparatus in accordance with claim 34 wherein said low-pressure fluid reservoir and said high-pressure fluid source are combined in a compressor.
 36. An apparatus in accordance with claim 33 including within said housing means an additional piston operable to define a variable-volume compression chamber adapted to serve as said first high-pressure fluid source and as said first low-pressure fluid reservoir.
 37. An apparatus in accordance with claim 33 including within said housing means displacer means defining high-temperature and intermediate-temperature chambers of variable volumes in fluid communication through heat storage means, said high-temperature and intermediate-temperatures in combination being adapted to serve as said first high-pressure fluid source and said first low-pressure fluid reservoir. 